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That is an absolutely textbook diagnostic result, and you just saved yourself from chasing ghosts in the RF decks. (And yes, those metal shell tubes run at finger-blistering temperaturesΓÇöwelcome to the 1940s!)
The fact that the voltage **increased** from +1.4V to +2.0V when you pulled the tubes is a massive, definitive clue. It completely exonerates your RF and IF tubes.
Here is exactly what that voltage rise means, and how we find the real culprit right now.
### Why the Voltage Went Up
Those RF and IF tubes were acting like a load. Because their grids were being fed a positive voltage, they were drawing grid current, which acted like a resistor pulling that positive leak down to +1.4V. When you removed the tubes, you removed the load. The leak was allowed to float up to its true, unloaded potential of +2.0V.
This proves the leak is coming from the **AVC generation circuit** itself, pushing its way down the AVC bus.
### The Next Diagnostic Step: The 6H6
The SX-28 uses an Amplified AVC system. An IF signal is amplified by the AVC Amplifier tube (V8, a 6AB7), and then fed into the AVC Rectifier tube (V9, a 6H6) to be turned into negative DC.
With those five hot RF/IF tubes still sitting safely on your bench (leave them out), and your scope/DMM still monitoring that +2.0V on the AVC line:
**Pull the V9 tube (the 6H6 AVC Rectifier).**
* **If the +2.0V instantly drops to 0V:** You have found the gateway. This almost guarantees that the coupling capacitor feeding the 6H6 is leaking. That capacitor connects the high-voltage plate of the V8 (6AB7) to the diode of the 6H6. When it leaks, B+ high voltage bleeds straight through the 6H6 socket and onto your AVC line. * **If the +2.0V is STILL there with the 6H6 removed:** The leak is physical. A terminal strip is compromised, a wire is chafed against a B+ line, or one of the paper AVC filter capacitors is physically touching a high-voltage terminal.
Pull V9 and let me know what the meter does!
Page: /Avc_test_and_repair/Gemini_904.shtml
Last modified: Monday, 20 Jul 2026